Journal of Magnetism and Magnetic Materials 190 (1998) 245—250 Low energy excitations in CeNiSn S. Raymond*, A.P. Murani, B. Fa  k, L.P. Regnault, G. Lapertot, P.A. Alekseev, J. Flouquet CEA-Grenoble, De & partement de Recherche Fondamentale sur la Matie % re Condense & e/SPSMS/MDN, 38 054 Grenoble Cedex 9, France Institut Laue Langevin, BP 156, 38 042 Grenoble Cedex 9, France ISSS, Kurchatov Institute, 12 3182 Moscow, Russia Received 14 April 1998; received in revised form 15 June 1998 Abstract We have studied the magnetic excitation spectrum of CeNiSn at low energies both on a polycrystalline sample using time-of-flight technique and on a single crystal with a triple axis spectrometer. The energy gap in the excitation spectrum is clearly observed in the polycrystalline sample reconciling the earlier discrepancies between the two kinds of measurements. The experimental results are consistent with the occurrence of a quasielastic signal within the gap without any significant wave vector dependence and characterized by an energy scale +0.2 meV.  1998 Elsevier Science B.V. All rights reserved. PACS: 75.20.H; 75.30.M; 78.70.N Keywords: CeNiSn; Kondo insulator; Neutron scattering 1. Introduction CeNiSn has attracted considerable attention in recent years as a novel system among Ce-based intermetallic compounds because of its non-con- ventional ground state. While most Ce-based inter- metallic compounds are good metals (for example, a Fermi liquid state is realized in the so-called heavy fermion (HF) compounds), CeNiSn is classi- fied as a ‘Kondo insulator’ [1,2]. It is now well * Corresponding author. Tel.: #33-4-76-883738; fax: #33- 4-76-885109; e-mail: raymond@drfmc.ceng.cea.fr. established that this behavior was partly due to impurity effects [3] and it is believed that the com- pound is a semimetal with a partially closed gap. A metallic behavior is indeed observed when the measurements are performed along the a-axis. The spin dynamics probed by NMR also reveals the pseudogap behavior which was interpreted with a V-shaped density of states [4]. The first inelastic neutron scattering (INS) experiments performed on time-of-flight (TOF) spectrometers using powder samples exhibited a quasielastic linewidth of the order of 3—4 meV at 10 K [5]. At the lowest temperature of 3 K, a broad inelastic contribution was observed at about 4 meV superposed to a quasi- elastic contribution [6]. Measurements performed 0304-8853/98/$ — see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 2 9 9 - 6